Veritas Storage Foundation 5.1 SP1 Advanced Features Administrator"s Guide (5900-1503, April 2011)

I/O temperature is a softer measure of I/O activity than access age. With access
age, a single access to a file resets the file's atime to the current time. In contrast,
a file's I/O temperature decreases gradually as time passes without the file being
accessed, and increases gradually as the file is accessed periodically. For example,
if a new 10 megabyte file is read completely five times on Monday and fsppadm
enforce runs at midnight, the file's two-day I/O temperature will be five and its
access age in days will be zero. If the file is read once on Tuesday, the file's access
age in days at midnight will be zero, and its two-day I/O temperature will have
dropped to three. If the file is read once on Wednesday, the file's access age at
midnight will still be zero, but its two-day I/O temperature will have dropped to
one, as the influence of Monday's I/O will have disappeared.
If the intention of a file placement policy is to keep files in place, such as on top-tier
storage devices, as long as the files are being accessed at all, then access age is
the more appropriate relocation criterion. However, if the intention is to relocate
files as the I/O load on them decreases, then I/O temperature is more appropriate.
The case for upward relocation is similar. If files that have been relocated to
lower-tier storage devices due to infrequent access experience renewed application
activity, then it may be appropriate to relocate those files to top-tier devices. A
policy rule that uses access age with a low <MAX> value, that is, the interval between
fsppadm enforce runs, as a relocation criterion will cause files to be relocated
that have been accessed even once during the interval. Conversely, a policy that
uses I/O temperature with a <MIN> value will only relocate files that have
experienced a sustained level of activity over the period of interest.
Prefer attribute
You can specify a value for the Prefer attribute for the <IOTEMP> and <ACCESSTEMP>
criteria, which gives preference to relocating files. The Prefer attribute can take
two values: low or high. If you specify low, Veritas File System (VxFS) relocates
the files with the lower I/O temperature before relocating the files with the higher
I/O temperature. If you specify high, VxFS relocates the files with the higher I/O
temperature before relocating the files with the lower I/O temperature. Symantec
recommends that you specify a Prefer attribute value only if you are using solid
state disks (SSDs).
See “Prefer mechanism” on page 379.
Different <PERIOD> elements may be used in the <IOTEMP> and <ACCESSTEMP>
criteria of different RELOCATE statements within the same policy.
The following placement policy snippet gives an example of the Prefer criteria:
<RELOCATE>
...
Administering SmartTier
File placement policy rules
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